1. Field of the Invention
This invention relates generally to an undesired signal canceller, and is directed more particularly to an undesired signal canceller for use in a ghost canceller and so on of a television receiver.
2. Description of the Prior Art
In the art, an undesired signal canceller shown in FIG. 1 is already proposed. In the example of FIG. 1, two antennas 1 and 2 are arranged apart with a distance d. The signal received by the antenna 1 is applied through a phase compensation circuit 3 with the phase shift of .phi.1 to a subtraction type circuit 4, while the signal received by the antenna 2 is supplied to the subtraction type circuit 4 through a variable phase shifter 5 with the phase shift of .phi.2.
In this case, it is known that the total directivity coefficient D(.theta.) of the above antenna device or arrangement is expressed as follows: ##EQU1## where k is a propagation constant and f(.theta.) is the directivity of each antenna.
Accordingly, in order to make a null point or null direction (which is a point when the receiving sensitivity is 0) on the direction angle .theta., it is sufficient if the following equation (2) is established. ##EQU2## In the above equation (2), if n=0 and .phi.2-.phi.1 is varied within the range of .+-.kd, the null point can be presented in a desired direction. Further, if .phi.2-.phi.1 is proportional to a frequency F(.phi.2-.phi.1=KF), k(=(2.pi.F)/(C), C is the velocity of light) is also proportional to the frequency F. Therefore, a null point .theta..sub.N is expressed as follows: ##EQU3## Thus, the null point .theta..sub.N can be set in a constant direction irrespective of received signal frequencies.
In this case, a coaxial cable is generally employed as the phase shifter 5 whose phase shift amount is proportional to the frequency F.
In this case, however, since the phase shift amount of the coaxial cable corresponds to its length, the phase adjustment thereof is very difficult. For this reason, a plurality of coaxial cables are prepared and they are switchably used. This adjustment, however, is rather complicated and it is also troublesome to change the null point after it has once been. Further, it is impossible to adjust the null point while a user stays near a television receiver.